Generally, when an aromatic compound is oxidized, the aromatic compound generates heat and an aromatic carboxylic acid and water are generated. Since the aromatic carboxylic acid is generally in a solid state, a solvent is injected such that the aromatic carboxylic acid does not exist in the solid state in a reactor. Since solubility of the aromatic carboxylic acid is high in a carboxylic acid, the carboxylic acid is injected into an oxidation reactor such that an aromatic oxide that is a reaction product maintains a liquid state. Accordingly, gas and liquid discharges generated after the oxidation contain much water and carboxylic acid, and thus it is necessary to perform a dehydration process so as to remove the water generated via the oxidation and recover the carboxylic acid back to the oxidation reactor.
In detail, processes for manufacturing a compound of a phthalic acid that is a type of aromatic carboxylic acid include an oxidation process for oxidizing an aromatic compound by air using a catalyst, such as a cobalt, manganese, or bromine, and a dehydration process for removing water and recovering an acetic acid that is a type of carboxylic acid used as a solvent in a reactor.
Here, a dehydration tower for recovering the acetic acid at a lower part and separating off the water at a top part is driven such that concentration of the acetic acid recovered at the lower part is generally from 90 to 95 wt %. Since a boiling temperature of a solution containing 90 to 90 wt % acetic acid at the lower part of the dehydration tower is from about 108 to about 111° C. under an atmospheric pressure, and a boiling temperature of the water at the top part of the dehydration tower is 100° C. under an atmospheric pressure, two or more dehydration towers may be used by setting a pressure difference between the dehydration towers to be about 1 kg/cm2 or lower. At this time, a condenser of the dehydration tower having a high pressure may operate as a reboiler of the dehydration tower having a low pressure, and thus the two or more dehydration towers may be driven via energy supplied to the reboiler of one dehydration tower even with a low pressure difference, thereby reducing energy based on a principle of multi-effect evaporator. The present invention is invented based on such features.
An acetic acid that is a type of carboxylic acid may be separated from water via conventional distillation or azeotropic distillation wherein an acetate compound or alcohol is circulated, and at this time, since a temperature of a lower part of a distillation tower is high, i.e., from about 125 to about 135° C., due to a pressure loss in a dehydration tower, a type of steam used in a reboiler is medium pressure steam (3.0 to 5 kg/cm2G and 143 to 158° C.).
Also, a lot of medium pressure steam is used to maintain concentration of an acetic acid in a discharge to be 0.5 wt % during conventional distillation and maintain concentration of an acetic acid in discharge to be 0.01 wt % during azeotropic distillation, so as to increase a recovery rate of the acetic acid and reduce waste water disposal costs.
FIG. 1 is a diagram for describing a method of recovering an acetic acid through conventional distillation.
Referring to FIG. 1, an apparatus for recovering an acetic acid through conventional distillation includes a dehydration tower 1, a reboiler, a condenser 3, and a condensate drum 4. A liquid stream having a low acetic acid concentration (acetic acid concentration: 40 to 70 wt %) and a gas stream having a high acetic acid concentration (acetic acid concentration: 70 to 88 wt %) are led into the dehydration tower 1, a portion of the acetic acid (acetic acid concentration: 88 to 95 wt %) from a lower part of the dehydration tower 1 is externally discharged while a remaining portion is led into the dehydration tower 1 again through the reboiler 2, and a non-condensate gas in a condensate that is from a top part of the dehydration tower 1 and passed through the condensate drum 4 selectively provided through the condenser 3 is externally discharged as a vent gas while some of the condensate is led into the dehydration tower 1 again as a reflux solution.
Consumption of medium pressure steam by a dehydration tower to dewater and recover an acetic acid through the conventional distillation is about 90 to 100 tons per hour in a factory generating 500,000 tons of phthalic acid per year.
FIG. 2 is a diagram for describing a method of recovering an acetic acid through azeotropic distillation.
Briefly describing with reference to FIG. 2, an apparatus for recovering an acetic acid through azeotropic distillation by using a conventional azeotropic agent includes an acetic acid recovering device 5 including the dehydration tower 1 for separating off an acetic acid and water through azeotropic distillation, the condenser 3 for condensing a gas discharged from the top part of the dehydration tower 1, an oil separator 4a for separating off an organic material (organic phase) and water (aqueous phase) of a liquid that passed through the condenser 3, the reboiler 2 for supplying steam to the dehydration tower 1, and an external azeotropic agent storage unit (not shown). The apparatus selectively includes an organic material recovering device 6 for recovering an organic material from a water phase stream of the acetic acid recovering device 5, an azeotropic agent recovering device 7 for recovering an azeotropic agent from an oil phase stream of the acetic acid recovering device 5, and an aromatic compound recovering device 8 for recovering an aromatic compound from the acetic acid recovering device 5. Detailed processes of the method of FIG. 2 are shown in FIG. 2.
Consumption of medium pressure steam by a dehydration tower to recover an acetic acid through azeotropic distillation is about 60 to 70 tons per hour in a factory generating 500,000 tons of phthalic acid per year, and total consumption of medium pressure steam is about 65 to 75 tons per hour when consumption of medium pressure steam by distillation towers operated to recover an azeotropic agent is added. Consumption of medium pressure steam during a method of recovering an acetic acid by using an azeotropic agent is less than that used during a method of recovering an acetic acid by using conventional distillation by about 25 to 30%, but a portion of the azeotropic agent may generate impurities by being lost in an oxidation reactor while a portion of the azeotropic agent cannot be completely prevented from being discharged with waste water.